1. Field of the Invention
The present invention relates in general to a discriminator apparatus for discriminating television broadcast program identifying signals. More particularly, the invention concerns a discriminator apparatus which can be applied to a television receiver of a multivoice system for discriminating the program identifying signals to determine whether a television program being currently broadcasted and to be received is in a stereophonic sound mode or in a heterogeneous voice mode (e.g. bilingual voice program in Japanese on one hand and a foreign language such as English on the other hand).
2. Description of the Prior Art
In the first place, description will be made of a hitherto known discriminator apparatus applied to a television receiver of a multivoice system in order to have a better understanding of the invention.
A television multivoice signal (multi-channel voice signal) exhibits such a frequency spectrum as illustrated in FIG. 1. More specifically, the multi-voice signal contains a main channel signal having a frequency distribution over a range of 50 Hz to f.sub.H where f.sub.H represents a horizontal synchronizing frequency of 15.750 kHz, a sub-channel signal which is obtained through frequency modulation of a voice or sound subcarrier of 2 f.sub.H (31.5 kHz) with a subsidiary sound or voice signal and has a frequency distribution over a range of f.sub.H to 3 f.sub.H (47.25 kHz), and a control signal obtained through amplitude modulation of a pilot carrier having a frequency of 3.5 f.sub.H (55.125 kHz) with a pilot signal having frequency of about 923 Hz or about 983 Hz.
In the case of a stereophonic television program broadcasting, a stereophonic sum signal (L+R) is transmitted as a main voice or sound signal, while a steroephonic difference signal (L-R) is transmitted as a subsidiary voice or sound signal. On the other hand, in the case of the mono-aural broadcasting, only the main voice or sound signal of the main channel is transmitted without being accompanied with the subsidiary or sound signal. Further, in the heterogeneous voice broadcasting mode such as bilingual programs in Japanese and English, for example, one of the voice signals is transmitted as the main voice signal with the other being transmitted as the subsidiary voice signal. Discrimination of the stereophonic broadcasting and the heterogeneous voice broadcasting from each other is effected with the aid of the pilot signal constituting the control signal. More specifically, in the case of a stereophonic television program, the control signal which is produced through amplitude modulation of the pilot carrier of 3.5 f.sub.H with the pilot signal f.sub.2 of about 983 Hz is transmitted, while for a heterogeneous voice program the control signal is transmitted which is produced by subjecting the pilot carrier of 3.5 f.sub.H to an amplitude modulation by the pilot signal f.sub.1.
The television receiver which is adapted to receive multivoice or multisound television broadcast programs includes pilot signal discriminator means destined for discriminating the pilot signals f.sub.1 and f.sub.2 from each other which identify the stereophonic television broadcast program and the heterogeneous multivoice television program, respectively.
Now, a typical one of the hitherto known discriminator apparatus will be reviewed by referring to FIG. 2 which shows in a block-diagram a main portion of a multivoice or multisound signal receiver circuit incorporating therein the prior art discriminator apparatus. In the figure, reference numeral 10 denotes an input terminal for the multivoice signal described above which terminal is coupled to a video detector of a television receiver through a voice frequency discriminator and a sound IF amplifier (not shown). When the multivoice signal received by the associated television receiver equipment is applied to the input terminal 10, the main channel signal of the multivoice signal is supplied to one of the input terminals of a matrix circuit 31 constituting a part of a demodulator circuit 30 through a de-emphasis circuit 21 which serves also as a filter, while the subchannel signal is supplied through a band-pass filter 22 to a frequency discriminator circuit 23 for being detected. Thus, there appears a sub-voice signal at the output of the discriminator circuit 23 which signal is then supplied to the other input terminal of the matrix circuit 31 through a stereophonic/heterogeneous voice program mode changing-over switch 32. In the stereophonic reception mode, the matrix circuit 31 produces stereophonic signals 2L and 2R at both output terminals thereof in a well known manner. The one stereophonic signal 2L is led to an output terminal 41 through a variable resistor 33 and a stationary contact a of a switch 35, while the other stereophonic signal 2R is led out to the other output terminal 42 through a stationary contact a of a switch 34, a variable resistor 33'a and a stationary contact a of a switch 35'. On the other hand, in the heterogeneous multi-voice reception mode, the matrix circuit 31 produces at one of the output terminals the main voice signal which is led out to the output terminal 41 through the same circuit path as for the stereophonic signal 2L. Further, the discriminator 23 produces the subsidiary voice signal at the output thereof, which signal is then led to the output terminal 42 through the de-emphasis circuit 24, a stationary contact b of the switch 34, a variable resistor 33'b, a stationary contact a of a switch 36 and a stationary contact b of the switch 35'. The switch 32 is normally opened and changed over the closed state by a switch change-over circuit 38 described hereinafter in the stereophonic reception mode. The switches 34, 35 and 35' are normally closed to the respective stationary contacts a. In the stereophonic sound reception mode, these switches 34, 35 and 35' remain closed to the respective stationary contacts a and are changed over to the respective contacts b in the heterogeneous reception mode under the control of a switch change-over circuit 39 described hereinafter. The switch 39 is normally closed to the stationary contact a. Since the operations of the demodulator circuit and the various switches in both of the stereophonic and the heterogeneous multi-voice reception modes are well known in the art, further detailed description will be unnecessary.
Next, discrimination of the pilot signals as well as the mode switching operation between the stereophonic reception and the heterogeneous multi-voice reception mode will be elucidated.
The pilot carrier signal f.sub.0 (55.125 kHz) of the multi-voice signal applied to the input terminal 10 is supplied to a pilot signal detector circuit 52 through a band-pass filter 51 having a passing frequency band of 15.125.+-.1 kHz. At the output terminals of the detector circuit 52, there appear the pilot signals f.sub.1 (ca. 923 Hz) and f.sub.2 (ca. 983 Hz) which are supplied, respectively, to low-frequency narrow fractional band mechanical filters 61 and 63 of a pilot signal discriminator apparatus generally denoted by a reference numeral 60. The signals having passed through the respective mechanical filters 61 and 63 are rectified and smoothed by succeeding rectifying and smoothing circuits 62 and 64, respectively, and thereafter supplied to the switch change-over circuits 39 and 38 of the demodulator circuit 30 which additionally includes the matrix circuit and the switch change-over circuit 37. When the stereophonic pilot signal f.sub.2 makes appearance at the output terminal of the mechanical filter 63 of the pilot signal discriminator apparatus 60, the switch change-over circuit 38 responds to the pilot signal f.sub.2 to thereby turn on the switch 32 of the demodulator circuit 30 of the receiver, whereby the demodulator circuit 30 is changed over to the stereophonic reception mode. On the other hand, when the pilot signal f.sub.1 for the heterogeneous multi-voice signal makes appearance at the output terminal of the filter 61 of the pilot signal discriminator apparatus 60, the switch change-over circuit 39 responds to the pilot signal f.sub.1 to change over the switches 34, 35 and 35' of the demodulator circuit 30 of the receiver to the respective stationary contacts b, as the result of which the demodulator circuit 30 is switched to the heterogeneous multi-voice reception mode. In the case of a mono-aural program, both of the subsidiary voice signal and the pilot signals disappear. Accordingly, by making use of this event information, it is possible to change over the switches 35, 35' and 36 to the respective stationary contacts b, whereby only the main voice signal is derived from the output terminals 41 and 42 of the demodulator circuit 30. For the discrimination of the mono-aural program, a fact that no pilot signal components are present may be made use of or alternatively a mono-aural voice discriminator designated by 65 in FIG. 2 may be employed. This discriminator 65 is adapted to discriminatively detect that the subsidiary voice signal or the pilot signals have a sufficiently small amplitude and produce a mono-aural voice program indicating signal V.sub.3 at the output thereof.
The prior art discriminator apparatus incorporated in the multi-voice system television receiver requires indispensably the use of the narrow fractional band mechanical filters for discriminating the program identifying signals, as will be appreciated from the above description. This mechanical filter has to be imparted with a passing band width not higher than about 10 Hz.sub.pp in view of the required noise-proof performance, which in turn means that severer requirements are imposed on the accuracy and stability of the center frequency (e.g. tolerance must not be greater than .+-.0.5%). Thus, the filter element utilizable in the low frequency band in the order of about 1 kHz is necessarily restricted to the mechanical filter exhibiting a high Q-value, involving eventually very expensive implementation.